1. Field of the Invention
The present invention relates generally to a solid image pick-up element employing, for example, CCDs (charge coupled devices), and more particularly to a solid image pick-up element capable of compensating for changes in characteristics with temperature.
2. Description of the Prior Art
FIG. 1 schematically depicts a conventional solid image pick-up element of the interline transfer type in which CCDs are used as light receiving units. This element is provided with a photoelectric transfer region 102 and an optical black region 103, both of which are formed on a substrate 100. A large number of light receiving units 101 are horizontally and vertically regularly arrayed on both the photoelectric transfer region 102 and the optical black region 103. The light receiving units 101 of the optical black region 103 are covered with a shielding film (shown by hatching) made of, for example, Al, AlSi or the like. The photoelectric transfer region 102 is provided for performing image pick-up whereas the optical black region 103 is provided for compensating signals detected by the photoelectric transfer region 102 for changes with temperature.
More specifically, at the optical black region 103, although incident light is shut off by the shielding film and the dark current is detected, the dark current is also affected by the change in temperature, as is the case with the signals detected by the photoelectric transfer region 102. Because of this, compensation is performed by subtracting the dark current from an output value detected by the photoelectric transfer region 102, thereby restraining the reduction of the dynamic range in vertical CCD registers (charge read units)104. Each of the vertical CCD registers 104 is interposed between two adjoining vertical rows of the light receiving units 101 or is formed on the left-hand side of the leftmost vertical row of the light receiving units 101, as viewed in FIG. 1. The vertical CCD registers 104 read and transfer signal charges generated in the light receiving units 101.
In the above-described conventional element, the shielding film is made comparatively thin, for example about 1 .mu.m thick, thereby preventing the generation of pin holes or the lowering in processability, which may be caused if the shielding film is made thick. After the formation of the shielding film, heat treatment such as, for example, H.sub.2 annealing is performed for the purpose of enhancing the quality of the substrate. This kind of heat treatment, however, occasionally makes crystals grow upon deposition thereof, thereby producing cracks in the shielding film. Such cracks allow light, though it is slight, to enter the light receiving units 101 of the optical black region 103, thus causing light leakage. Furthermore, at the optical black region 103, a silicon oxide insulation film 106 is interposed between the substrate 100 and the shielding film 105, and the upper surface thereof has undulations, for example, across a height of about 1 .mu.m at regions A, as shown in FIG. 2. The presence of such undulations lower the covering state of the shielding film 105, thereby occasionally causing light leakage at the regions A. If the light leakage occurs at the optical black region 103, the dark current value varies and correct temperature compensation cannot be performed.
It is conceivable that plural layers of shielding films are superimposed one upon another to prevent light leakage. This structure, however, cannot prevent the occurrence of pin holes or light leakage from between metallic grains. Furthermore, the formation of the plural layers of shielding films inevitably needs additional processes.